Caustic solution treatment process

ABSTRACT

A method for treating a spent caustic solution having a pH greater than about 10.5 includes the steps of (a) lowering the pH of the caustic solution into the range of between about 10 to 10.5 to produce a pH-modified caustic solution; (b) oxidizing the pH-modified caustic solution until the solution to produce a non-sulfide reactive oxidized solution; and (c) lowering the pH of the oxidized solution to a pH less than 9. Methods for oxidizing inorganic substrates in aqueous solutions using an oxidizing agent such as pure oxygen, oxygen-enriched air or ozone are improved by including in the headspace above the aqueous solution a diluent gas and by recycling the oxygen with diluent gas into the reactor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 10/456,096,filed Jun. 6, 2003, now U.S. Pat. No. 7,005,076, incorporated herein byreference as if set forth in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

In the petroleum and petrochemical industries, acid gases containingCO₂,H₂S, and VOCs are commonly scrubbed with aqueous caustic solutionsthat can contain, for example, NaOH. Such scrubbing processes yield ahazardous spent caustic solution having a high pH, typically greaterthan 10.5, and sulfide reactivity. Spent caustic solutions are alsocharacterized by high chemical oxygen demand (COD) and biochemicaloxygen demand (BOD). BOD and COD are measures of oxygen uptake by waterarising from chemical and biodegradable agents in the water. The COD andBOD exerting compounds in spent caustic solutions can include, but arenot limited to sodium sulfide (Na₂S) sodium hydrosulfide, and variousorganic compounds, including naphthenic acids and the like.

Typically, sulfides in the spent caustic solutions are oxidized using agaseous oxidant such as air or oxygen to convert the sulfides intothiosulfates and sulfates which can be subsequently neutralized usingacid. Unfortunately, conventional oxidation can result in severe foamingproblems, rendering the air oxidation process inoperable withoutexpensive defoaming chemicals. Conventional efforts to control pH beforeoxidation by, e.g., adding sulfuric acid, undesirably evolves hydrogensulfide gas.

The art is in need of alternative methods for treating spent causticsolutions to avoid foaming and hydrogen sulfide evolution while yieldinga waste discharge stream having reduced BOD and COD. It is alsodesirable to reduce the amount of organic matter present in treateddischarge streams.

BRIEF SUMMARY OF THE INVENTION

The present invention is summarized in that the problems encountered inthe prior art are overcome in a method for treating a spent causticsolution having a pH greater than 10.5 includes the steps of loweringthe solution pH to about 10.5 using carbon dioxide, oxidizing thepH-adjusted solution until non-sulfide reactive in a US EPA sulfidereactivity test, then further lowering the pH of the oxidized solutionto less than about 9.0.

It is an advantage of the present invention that the initial lowering ofthe pH permits oxidation without foaming.

It is another advantage of the present invention that the initiallowering of the pH with carbon dioxide does not result in evolution ofH₂S gas.

It is yet another advantage of the invention that after treatmentaccording to the method, the treated spent caustic solution hassubstantially lower BOD and COD as a result of lower sulfidic oxygendemand.

It is yet another advantage of the invention that the final treatedspent caustic solution is not sulfide reactive.

In a related embodiment, a method is provided for removing oily organicmatter that can separate from the spent caustic solution duringneutralization before or after oxidation, when the pH of the solutionfalls in the range of about 9.5 to about 10.5. When this occurs, oilyorganic matter separates from and floats on the organic phase. In accordwith the preferred embodiment, the floating oily organic matter can beremoved, thereby further lowering the COD and BOD of the spent causticsolution.

Further, solids that can precipitate during oxidation and lowering ofthe pH can also be filtered to generate a final discharge stream havinglow levels (about 20 mg/liter or less) of suspended solids.

Finally, VOCs stripped from the solution during oxidation can be removedby, passing the solution through a granular activated carbon bed or byburning.

It is another advantage of the present invention that it can bepracticed at ambient temperature and pressure.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detaileddescription.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic depiction of one embodiment of a process accordingto the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In accord with the present invention, a spent caustic solution is firstpartially neutralized using an acidic gas, preferably a weak-acid gas,such as carbon dioxide, to a pH in the range of approximately 10 to10.5. Alternatively, acid gasses, such as sulfur dioxide, hydrogensulfide, and the like may also be used. Liquid acids, such as HCl andH₂SO₄ are not suited for use at this stage because of the potential onmixing for localized low pH hot spots that can increase the potentialfor generating gaseous H₂S.

During this partial neutralization stage, evolved gas can be vented fromthe system.

After partial neutralization, the pH-lowered solution is oxidized atambient temperature and pressure to substantially reduce the sulfidicoxygen demand of the solution, while also lowering the pH by removingits sulfidic alkalinity. At the same time, the solution is renderednon-sulfide reactive, as determined by the US EPA Hazardous WasteToxicity Characteristic for Sulfide Reactivity. The oxidation step canuse oxygen, ozone, air, and air enriched with oxygen. The use ofenriched air or pure oxygen reduces or eliminates gaseous discharge fromthe treatment system, thereby minimizing the emission controlrequirements. However, when air is used, organic compounds stripped fromthe caustic solution may be scrubbed in an oxidizing solution(containing, e.g., H₂O₂) or adsorbed in a granular activated carbon bed,or sent to flare for combustion.

After oxidation, further pH control is undertaken as needed to lower thesolution pH to desirable levels below about 9.0. The post-oxidation pHcontrol can be accomplished by contacting the solution with additionalCO₂ or other acid gases, or with strong mineral acids such as H₂SO₄,H₃PO₄, HCl, HNO₃ and the like. Addition of a strong acid will liberatecarbon dioxide added during the initial partial neutralization step. Theliberated CO₂ can be reused in subsequent initial partialneutralization.

During the pH control steps before or after oxidation, a substantialamount of oily organic matter can separate from the aqueous phase at apH in the range of approximately 9.5 to 10.5. Preferably, the floatingoily organics are removed from the waste to further reduce the COD andBOD of the treated waste. Precipitated solids generated during oxidationand neutralization can also be filtered to generate a low-suspendedsolids waste discharge stream.

EXAMPLE

A spent caustic solution was treated with gaseous CO₂ in a bench-scalelaboratory treatability study at various levels of pH control. The spentcaustic solution was characterized as follows:

pH 13.9 Sulfide 110 g/L as Na₂O Sodium 4.9 weight % Acid Oils 5 weight %Specific gravity 1.099

The following results were obtained:

NO CO₂ NEUTRALIZATION NEUTRALIZATION BUBBLING TO pH ~10 TO pH <9.5 pH13.9 10.1 9.3 H₂S No No Yes present in head space (measured by Draegertube) Foaming Yes No — during oxidation With air bubbling

Neutralization of the spent caustic solution with dilute H₂SO₄ wascharacterized by the presence of H₂S in the head space.

The COD of the spent caustic solution aqueous layer measured by HACHmethod at various levels of treatment were as follows:

Screening COD Treatment level (mg/L) Untreated 147,000 Neutralization(Step 1) with CO₂ to pH 9.8-10 72,900 Separation of oily layer (Step 2)with air oxidation of 35,700 Aqueous layer (Step 3) which was filteredafter oxidation Acidification with H₂SO₄ to pH <9.0 (Step 4), removeoily 18,300 Layer, filter aqueous layer

The acidification of the air oxidized solution in Step 4 showed nopresence of H₂S in the head space.

1. A method for oxidizing a spent caustic solution, the methodcomprising the steps of: lowering the pH of the spent caustic solutioninto the range of between about 10 to 10.5 to produce a pH-modifiedcaustic solution; mixing the pH-modified caustic solution with anoxidizing gas selected from the group consisting of pure oxygen,oxygen-enriched air and ozone in a reactor at ambient temperature andpressure to produce a non-sulfide reactive oxidized solution andheadspace gas; lowering the pH of the oxidized solution to a pH lessthan about 9; diluting the head space gas with a diluent gas; andrecycling the head space gas with the diluent gas into the reactor. 2.The method of claim 1 wherein the head space gas is recycled via aneductor in an untreated wastewater stream.
 3. The method of claim 1wherein the head space gas is recycled via an eductor in a recycledwastewater stream.
 4. The method of claim 1 wherein the diluent gas isselected from the group consisting of nitrogen, carbon dioxide and air.